This invention relates to the field of integrated circuits, and more particularly to an input buffer for digital integrated circuits used in environments where signal noise is common.
Integrated circuits are widely used in applications involving computers, networks, and their related peripherals, such as disk drives, CD-ROM drives, printers, scanners, and modems. The number of installed computers continues to increase as well as the number and types of peripheral devices.
Peripheral devices must interface with a computer in order for the them to communicate and transfer information. Connections between computers and their related peripherals are frequently made using cables because, among other reasons, the cost of cable connections is relatively inexpensive. Moreover, long cable runs are often used because it is usually impractical to have a computer and its related peripherals in close proximity to another.
In contrast, on printed circuit boards, integrated circuits and other components are a short distance from one another and relatively short metal traces connect the different components. These metal traces generally exhibit good signal quality as compared to long cable runs which usually have a much noisier signal quality. Noise is introduced into the environment from the cable itself, the connectors and terminators of a cable, reflections from cable terminations, noisy or faulty integrated circuits, as well as many other sources.
Noise is inherent and causes problems because integrated circuits may interpret the noise as a valid logic level and pass this incorrect information on to other circuitry on the same integrated circuit or to other integrated circuits in the digital system. In the past, device manufacturers have emphasized solving this problem by slowing an edge rate at an output of an integrated circuit to reduce the introduction of noise into the environment. However, this only addresses one source of the noise and does not solve the problem completely. Other sources, such as noise from the cables themselves or reflections within the cables still contribute a great deal of noise and cause communication problems. Furthermore, in order to function reliably, a system must not only reduce transmission-line noise at an output, but must also be able to deal with unfiltered noise when receiving data at an input because noise reflections are present on both transmitting and receiving phases.
Another problem is that integrated circuits used in a computer or network environment generally have an input skew, a significantly slower rising edge time than falling edge time. This skew is especially pronounced when open-drain output drivers are used because a pull-up of the rising edge is accomplished using a resistor. This is a problem for integrated circuits because unless the skew is corrected, the propagation delay through an entire integrated circuit will also be significantly skewed, which is undesirable.
Typically, inverting buffers are used for the input stage of an integrated circuit, but these propagate most types of noise occurring at an input pad of the input stage through to the internal circuitry of the integrated circuit. An input stage having inverting buffers has only very slight noise immunity. Furthermore, inverting buffers are limited in their ability to correct input skew.
Schmitt trigger circuits also have been used in the industry. Used in the input stage, a Schmitt trigger circuit has more noise immunity than a simple inverting buffer input stage, but nevertheless, this noise immunity remains similarly limited. Furthermore, a Schmitt trigger, like inverting buffers, does not address the input skew problem.
As the number of computer and network related installations and applications and peripherals continues to increase, a demand for digital integrated circuits increases. Hence, there is a need to implement better techniques in the design of digital integrated circuits to deal with the noise and input skews present in these environments.
The present invention includes an input buffer for an integrated circuit and provides reduced sensitivity to the input noise present in a transmission line environment. The input buffer of the preferred embodiment also deskews an input signal having a rise time that is much slower than a fall time, such as that from an open-collector output driver, so that a rising edge propagation delay and a falling edge propagation delay of the input buffer are approximately equal.
In particular, the present invention is an input buffer for an integrated circuit for receiving an input signal from a passive pull-up output driver, the input signal having a rise time slower than a fall time, including: a Schmitt trigger having (1) an input, coupled to an input pad of the integrated circuit, for receiving the input signal, (2) a pull-up driver, coupled to the input, having a first drive capability due to a first gate size, and (3) a pull-down driver, coupled to the input, having a second drive capability due to a second gate size, wherein the second gate size is from five to fifteen times greater than the first gate size. Further, the output of the Schmitt trigger is coupled to a buffer having an input gate size from five to fifteen times greater than the first gate size of the Schmitt trigger.
Further features of the invention, its nature and various advantages will become more apparent from the accompanying drawings and following detailed description of the preferred embodiments.